Living polymerizations can provide advantages over other polymerization techniques, such as well-defined polymer structures and low degree of compositional heterogeneity. Many of the variables that affect polymer properties can be controlled, including molecular weight, molecular weight distribution, copolymer composition and microstructure, stereochemistry, branching and chain end functionality.
Living anionic polymerization of styrene and diene monomers were first described by Szwarc and his coworkers. See M. Szwarc, Nature 178, 1169 (1956) and M. Szwarc, et al., J. Am. Chem. Soc. 78, 2656 (1956). While living anionic polymerization can be effective for the controlled polymerization of non-polar monomers, anionic polymerization of polar monomers, such as methacrylates and acrylates, is more problematic. The presence of a carbonyl group in acrylate monomers complicates anionic polymerization of polar monomers. For example, nucleophilic attack at the carbonyl group can lead to no initiation or polymerization termination.
Various techniques have been proposed to address the problem of anionic polymerization of methacrylate and acrylate monomers. Proposals include low polymerization temperatures (-78.degree. C.), the use of sterically hindered initiators, bulky alkyl ester groups, and the addition of complexing agents, such as crown ethers, lithium chloride and lithium alkoxides. Other techniques include metal-free anionic polymerization using delocalized carbanion initiators with nonmetallic tetrabutylammonium salts (see, e.g., M. T. Reetz, Angew. Chem. Int. Ed. Eng. 27, 994 (1988)); group transfer polymerization, using a silicon-based initiator (O. W. Webster, et al., European Patent 0 068 887 (1986)); and immortal polymerization using aluminum porphyrins as initiators (M. Kuroki et al., J. Am. Chem. Soc. 109, 4739 (1987); Y. Hosokawa, et al., Macromolecules 24, 824 (1991)). See also, T. P Davis, et al., Rev. Macromol. Chem. Phys., C34(2), 243-324 (1994) and H. Hsieh and R. Quirk, Anionic Polymerization (Marcel Dekker, Inc., New York 1996) for a more complete review.
Although useful, these and other techniques of anionic polymerization of methacrylate and acrylate monomers can suffer from drawbacks, such as ineffectiveness at higher temperatures, slow reaction rates, broad molecular weight distributions, poor copolymerization with polar and non-polar comonomers, and the like. Further, these processes can be expensive, thus limiting their commercial applicability. These problems can be compounded when polymerizing acrylate monomers, which are more reactive than methacrylate monomers.